For LCD devices, a classification based on the operating mode of liquid crystal molecules includes modes such as PC (Phase Change), TN (Twisted Nematic), STN (Super Twisted Nematic), ECB (Electrically Controlled Birefringence), OCB (Optically Compensated Bend), IPS (In-Plane Switching), VA (Vertical Alignment), FFS (Fringe Field Switching) and FPA (Field-induced Photo-reactive Alignment). A classification based on the driving mode of the device includes PM (passive matrix) and AM (active matrix) types. The PM types are classified into static type, multiplex type and so forth, and the AM types are classified into TFT (thin film transistor) types, MIM (metal-insulator-metal) types and so forth. The TFT types are classified into amorphous silicon types and polycrystal silicon types. The latter are classified into a high-temperature type and a low-temperature type depending on the production process. A classification based on the light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight, and a semi-transmission type utilizing both natural light and a backlight.
The LCD device includes a liquid crystal composition having a nematic phase. This composition has suitable characteristics. An AM device having good characteristics can be obtained by improving of the characteristics of this composition. Table 1 below summarizes the relationship between these two groups of characteristics. The characteristics of the composition will be further explained on the basis of a commercially available AM device. The temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is about 70° C. or higher and a desirable minimum temperature of the nematic phase is about −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying moving images on the device. Response time that is one millisecond shorter than that of the other devices is desirable. Thus, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1Characteristics of Composition and AM DeviceGeneral Characteristics ofGeneral Characteristics of AMNo.CompositionDevice1Wide temperature range of aWide temperature range innematic phasewhich the device canbe used2Small viscosity1)Short response time3Suitable optical anisotropyLarge contrast ratio4Large positive or large negativeLow threshold voltage,dielectric anisotropylow power consumption, andlarge contrast ratio5Large specific resistanceLarge voltage holding ratio,and large contrast ratio6High stability to UV light and heatLong service life1)A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
The optical anisotropy of the composition relates to the contrast ratio of the device. A large optical anisotropy or a small optical anisotropy, namely a suitable optical anisotropy, is necessary depending on the mode of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the type of operating mode. This value is in the range of about 0.30 μm to about 0.40 μm for a device having a VA mode, and in the range of about 0.20 μm to about 0.30 μm for a device having an IPS mode or an FFS mode. In these cases, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, low power consumption and a large contrast ratio of the device. A large dielectric anisotropy is thus desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. It is thus desirable that a composition should have a large specific resistance at a high temperature as well as at room temperature in the initial stages. It is desirable that a composition should have a large specific resistance at a high temperature as well as at room temperature, after it has been used for a long period of time. The stability of the composition to UV light and heat relates to the service life of device. A device has a long service life if the stability is high. Such characteristics are desirable for an AM device used for a liquid crystal projector, a liquid crystal television and so forth.
A liquid crystal composition including a polymer is used for a LCD device of a polymer sustained alignment (PSA) type. First, a composition to which a small amount of polymerizable compound has been added is poured into a device. Next, the composition is irradiated with UV light, while a voltage is applied between the substrates of this device, to polymerize the polymerizable compound and form a network structure of a polymer in the composition. In this composition, the polymer makes it possible to adjust the orientation of liquid crystal molecules, and thus the response time of the device is decreased and image burn-in is reduced. Such effect of the polymer can be expected for a device having a mode such as TN, ECB, OCB, IPS, VA, FFS or FPA.
A composition having positive dielectric anisotropy is used for an AM device having a TN mode. A composition having negative dielectric anisotropy is used for an AM device having a VA mode. A composition having positive or negative dielectric anisotropy is used for an AM device having an IPS mode or an FFS mode. A composition having positive or negative dielectric anisotropy is used for an AM device with a PSA type. With regard to the device of a PSA type, examples of the liquid crystal composition having negative dielectric anisotropy are disclosed in the following Patent documents No. 1 to No. 6.
Patent document No. 1: JP 2003-307720 A.
Patent document No. 2: JP 2004-131704 A.
Patent document No. 3: JP 2006-133619 A.
Patent document No. 4: EP 1889894 A.
Patent document No. 5: JP 2010-537010 A.
Patent document No. 6: JP 2010-537256 A.
An improvement of characteristics such as image burn-in is required to a liquid crystal composition, since the performance of a PSA type device has been improved recently. It is considered that a polymerizable compound in which the molecular structure is rod-like has a high ability to orient liquid crystal molecules. In contrast, it has a poor solubility in the composition and a large amount of the polymerizable compound cannot be added. If a polymerizable compound having a high solubility in a liquid crystal composition could be found, it is possible to give a large pretilt angle to liquid crystal molecules, and thus the response time of the device is decreased and image burn-in is reduced.